Vixen LVW 42mm with 65° or 72°

[GotGazer]

Hi,

I ordered a Vixen LVW 42mm 72° from an online shop.

It arrived yesterday but there is a 65° AFOV printed on the eyepiece. 

I've seen this before in other shops mixing 72° with 65°. The item description says 65° but the picture shows 72° and vice versa. Why is it like this? A new generation mixed with an old "in-stock" prooduct? Or is it the very same product just relabeled with another AFOV value?

I'm asking because I want to be sure to get a 72° AFOV which would give me a TFOV close to maximum of my C9.25 scope as far as I understand.

In my case the online shop clearly says 72° AFOV. Should I return the eyepiece?

[alan potts]

This is something that cropped up before over a year ago and I am not sure we ever got to the bottom of the question. I have a Televue 41mm and that has a 68 degree FOV, now TV being who they are have always pushed the limit of the envelope. Myself I find it hard to accept that a company albeit a very good one can out Tele-vue Tele-vue, with 1 more millimeter and 4 more degrees, without vignette issues.

The 41mm in fact show a mild vignette when used in my LX 12 inch scope with a diagonal, the one it was bought for. The limit of what can be seen as you probably know is governed by the size of the field stop of the eyepiece, this must be within a very tiny amount identical on both eyepieces in order to fit the telescopes. If the Vixen is 72 degrees and 42mm I struggle to understand how they do it. I am not saying this is not the case but personally I do not believe it.

I have even considered buying one to see for myself.

If there are like eyepieces (65) from your supplier and you ordered  72 degrees based on a picture then I for one would not be happy with what you have received, my advice is send it back and make sure they pay for the postage, it's their mistake.

Alan

[YKSE]

My guess is that 65° AFOV is more correct than 72°.

The Field Stop(FS) of 42 LVW is measured as about 46.57mm by a CN member (he has loads of eyepieces), that is the widest of ANY 2" eyepieces, so it gives the widest possible TFOV of a 2" eyepiece.

Or we can do some math, TFOV can be calculated in two ways:

1. TFOV = AFOV / magnification

2. TFOV = FS *57.3/ FLscope

we get

AFOV/magnification = FS *57.3/FSscope

that leads to

AFOV = FS *57.3*magnification/FSscope =FS * 57.3 /(FSscope/magnification)

and magnification/FSscope is actually focal length of the eyepiece, that gives

AFOV = FS * 57.3 / FLeyepiece = 46.57*57.3/42=63.53

And that CN member measure 63.5°

[michael.h.f.wilkinson]

My LVW 42 has 72° on the barrel. This may be correct in terms of apparent field of view, i.e. the size of the image circle seen through it. This is different from what is sometimes confusingly called the true apparent field of view, which is computed from the field stop size as above. The difference is accounted for by pin-cushion distortion.

[GotGazer]

Seems that my question was more difficult to answer than I thought.

I spent another ½h trying to get a clear answer but I'm only getting more confused.

So my guess this far is that YKSE is right about his (here we go again) "guess" about a more correct AFOV that would be 65°.

Then it's just a relabelling of the product from 72° to 65°. And some sellers are still using an old 72° AFOV item specification just to make the product more desirable.

Anyone else? 

[ronb]

I have an interest in this discussion as I am trying to talk myself into getting one. Although some sellers are quoting 72deg most sites, including Vixen's, state 65 deg.

[YKSE]

I don't think it'll make much difference if it's marketed as 65° or 72°, the physical limit of Field Stop(FS) determines the FOV you can see in a eyepiece.

2" eyepiece has outer diameter 50.8mm, with wall thickness of 2mm (measured from my 2" eyepieces) of the 2" nosepiece, the inner diameter is 46.8mm, and the Field Stop will be even slightly smaller than that, so the measured FS of 46.57mm is about as big as physically possible.

As Michael mentioned above, the pincushion in the eyepiece may account for the discrepancy, but don't make any real world difference.

[Mr Spock]

Here's mine:

[Ruud]

Vixen says 65° on their website. That's what GotGazer 's says on the barrel, but Michael's says 72°.

You'd think that Vixen reduced the afov without changing the name of the eyepiece. That's confusing!!

Pincushion plays a role in how big the true fov is. The 10 mm 90° Takahashi UW (the king of pincushion distortion) has a bout the same true field as the 10 mm 82° Celestron Luminous.

[michael.h.f.wilkinson]

I just compared the image circles of the Delos and the LVW 42, and visually they appear identical (if anything, the LVW is a touch wider). This is just the apparent angular dimension. This is usually the figure quoted. In practice, the field stop dimension is more important. What may have happened is that Vixen decided to start quoting this geometric apparent FOV.

[Mr Spock]

Whatever it is, enjoy it cause it's like looking though an open window

[michael.h.f.wilkinson]

It is a nice EP. It is handy for those (rare) occasions when the 31T5 "Panzerfaust" doesn't frame the object well.

[pete_gamby]

I could probably have put this to bed sooner - sorry for being late to the discussion. Here is what Vixen Japan told me when I asked them about this some time ago:

"Once a wrong field of view specification was indicated on the #3727 by mistake and it was corrected to a 65 degree FOV later."

So, just a typo on the eyepiece. Quite an important typo in many people's view I'm sure...

HTH

Pete

[michael.h.f.wilkinson]

It is an odd coincidence that the old special on the barrel is exactly the same as the apparent FOV. I think they changed it for the reasons given earlier.

[Mr Spock]

I can confirm looking through the following eyepieces:

Radian 60° - has a wider view than a Plössl, 50°

LVW 65° - has a wider view than a Radian

42mm LVW 72° - has a wider view than an LVW

Nagler 82° - has a wider view than a 42mm LVW

The differences are evenly spaced

[alan potts]

I could probably have put this to bed sooner - sorry for being late to the discussion. Here is what Vixen Japan told me when I asked them about this some time ago:

"Once a wrong field of view specification was indicated on the #3727 by mistake and it was corrected to a 65 degree FOV later."

So, just a typo on the eyepiece. Quite an important typo in many people's view I'm sure...

HTH

Pete

If this is the case it will no doubt be exactly the same as my 41mm 68 degree Panoptic which was what I thought all along. I am not just saying this because I have mainly Tele-Vue but I just can't see that they would be outdone with the reputation they have as they always seem to go that extra one, you could say they go to 11.

Alan.

[GotGazer]

I could probably have put this to bed sooner - sorry for being late to the discussion. Here is what Vixen Japan told me when I asked them about this some time ago:

"Once a wrong field of view specification was indicated on the #3727 by mistake and it was corrected to a 65 degree FOV later."

So, just a typo on the eyepiece. Quite an important typo in many people's view I'm sure...

HTH

Pete

Thanks Pete!

[JeremyS]

Interesting (if old) discussion - I wondered my my LVW 42mm eyepiece says 72° on the barrel!

[Don Pensack]

If you have one labeled 72°, it's a "collector's item".

It's like the 2" Olivon 70° eyepieces with 1.25" on the barrels.  

Oops. 

Like a double-stamped coin, or some cars a few years ago recalled because they had a different name badge on the rear from the model they were.